Google has confirmed that its fleet of self-driving cars have been involved in 11 accidents during the programme's six-year life, but not once were its software or drivers at fault.
Yesterday we reported that since September 2014 there have been four accidents involving autonomous cars in the state of California.
One of those vehicles was registered to Delphi, a top-tier automotive supplier, while the other three cars belonged to Google. All four accidents occurred at low speeds (below 16km/h) and in all four cases the autonomous car, or its human driver, were deemed to be not at fault.
In an extensive piece for Medium, Chris Urmson, the head of Google's self-driving car project, has laid out some of the facts behind these incidents, as well as how the Google's autonomous driving software works.
Urmson reveals that in the six-year lifespan of the project, the company's self-driving cars have been involved in 11 accidents on public roads. All incidents took place at low speeds and, he says, "not once was the self-driving car the cause of the accident".
According to Urmson, Google's modified Lexus RX SUVs were rear-ended seven times, side-swiped a few times and hit once "by a car rolling through a stop sign".
In Urmson's view: "Even when our software and sensors can detect a sticky situation and take action earlier and faster than an alert human driver, sometimes we won’t be able to overcome the realities of speed and distance; sometimes we’ll get hit just waiting for a light to change."
Google's self-driving car czar also argues that while 11 accidents in six years with a fleet of around 20 cars may sound high, that's not necessarily the case because the "most common accidents our cars are likely to experience in typical day to day street driving — light damage, no injuries — aren’t well understood because they’re not reported to police".
In the USA, the country's National Highway Traffic Safety Administration (NHTSA) estimates that these type of low speed, low impact crashes account for around 55 percent of all accidents.
Since the programme began, Google has built up a fleet of 20-plus autonomous vehicles. Collectively these cars have clocked 2.7 million kilometres, with around 1.6 million of those kilometres completed with the car controlling itself.
In California, registered autonomous vehicles are required to have a trained and accredited person sitting in the driver’s seat, who can take charge if the software isn't able to cope or if there's a serious bug in the system. Urmson says that its safety drivers have noticed not only "people weaving in and out of their lanes [but also] people reading books, and even one playing a trumpet".
Unlike humans who can be temporarily distracted or just wilfully negligent, Urmson says that his team's cars have "360-degree visibility and 100 percent attention out in all directions at all times; our newest sensors can keep track of other vehicles, cyclists, and pedestrians out to a distance of nearly two football fields".